Fooling Around

Although I personally have no experience with tricky DirectShow filters that decide on possibility of connection not only looking at media type and other capabilities, there has been a number of cases mentioned that certain video decoders will only connect to renderers or video renderers in order to avoid interception of decoded data.

It was recently mentioned that one of such filters checked Misc Flags obtained through IAMFilterMiscFlags interface to make sure it is connected to renderer downstream. Before we proceed, let us make it clear that it is silly and can only protect from beginner, a complete newbie. At the very least it should have checked peer filter’s CLSID and compare against white list of stock video renderer: Video Renderer, Video Mixing Rendeder 7, Video Mixing Rendeder 9, Enhanced Video Renderer. A custom video renderer or even just a transformation filter with a renderer flag would be able to receive decoded data and make it available for any further processing.

What I am going to do is to take Sample Grabber Filter as a base and make a new filter from it which will only connect to renderer (it will check the flags as described above). Then another filter, which will also use Sample Grabber Filter base, will accurately fool the first one and connect to output of the first filter and will render video to complete the graph.

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See beginning in microsoft.public.win32.programmer.directx.video newsgroup.

This sample is demonstrating COM aggregation to embed an existing filter an re-expose it as a new filter having inner filter pre-initialized.

The Visual Studio C++.NET 2008 projects contains a DirectShow filter class that registers itself under Video Capture Sources category and embeds File Source (Async) Filter inside initialized to stream clock.avi file from Windows directory.

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